Such a method and such a device are generally known in the art and are, for example, used in electrical systems to protect people and/or the system itself, for instance, from the destructive effects of an arc caused by a short circuit or to mitigate its effects at the very least. For example, animals or even dropped tools and (damp) dirt can reduce the insulation or spark gap between two conductors having different voltage potentials such that an arc is formed. The very high currents resulting in some cases can lead to powerful explosions due to the air heating up in a very short space of time.
Electrical systems are therefore often monitored to prevent such an arc occurring, and in many cases this is achieved by measuring the current flowing through the electrical conductors and detecting the extremely intensive light resulting from an arc. If both criteria are fulfilled, an alarm signal or a switching signal is issued to close a switch between said conductors having different voltage potentials. On the one hand, this causes the arc to be extinguished quickly; on the other hand, it also causes very high currents to occur in the supply lines, which trip a higher-level circuit breaker, which ultimately isolates the danger area from the network. Of course, said switching signal can also be used to directly open a switch or a plurality of switches in the supply lines leading to the arc.
In principle, there are a number of publications on this subject in the prior art. For example, U.S. Pat. No. 6,229,680 B1 discloses a system for optical arc detection in which the light received by a measurement point is split and passed through two different band-pass filters. If the difference between the electrical signal resulting from the received light and the electrical signal resulting from a background light exceeds a threshold value, an arc detection signal is issued.
EP 1 538 722 A2 also discloses a system for arc detection in which the light received by a measurement point is combined with a modulated reference light source and then split and passed through two band-pass filters. If a first electrical signal, which represents light at a first wavelength, exceeds a second electrical signal that represents light at a second wavelength excluding the modulated reference light, an alarm is issued.
CH 676174 A also discloses the use of a band-pass filter to limit the light received by a measurement point to an arc wavelength range.
Furthermore, DE 10 2010 016 036 A1 discloses a color sensor comprising a red filter, a green filter and a blue filter for detecting an arc.
Finally, GB 2477970 A discloses an optical fiber having a fluorescent material which transforms light in a wavelength range that is characteristic of an arc into light that has a different wavelength. This transformed light is then passed to an optical sensor.
As a general rule, it is difficult to detect a dangerous, short-circuiting arc with any degree of certainty using methods in the prior art. This is because, in electrical systems, arcing can occur even during normal operation, for example in the form of switching arcs when disconnecting a live switching contact. The arc in a switching device itself is admittedly not directly visible, although luminous gas can be emitted by the switching device, said gas containing at least metal vapor which originates from the burning switching contacts, or arc deflectors or arc splitters, in the switching device. In addition, the gas may also contain other constituents, for example evaporated plastics from the housing of the switching device.
This luminous gas may erroneously be interpreted as dangerous arcing, leading to an electrical system which is actually working normally being shut down unnecessarily. In the prior art, this kind of malfunction can in particular be observed if a phase-to-earth short circuit (without arcing or with arcing outside the switch cabinet) causes a very high measurable current in the supply line and also causes an associated circuit breaker to trip. The luminous gases emitted by the circuit breaker seem to fulfil the two conditions (high current and bright light) mentioned at the outset for a dangerous arc.